slow twist AR barrel?

[Gswain]

I've been thinking about building an AR, and i would like to cast for it if im gonna build it. From the research I've been able to do here, a slow twist barrel is a necessity, otherwise the barrel will lead horribly. The slowest twist I have been able to find on midwayusa is a 1 in 10" made by olympic, and it is quite spendy. Anyone know of a good source to pick up a barrel for an AR that is closer to the 1 in 12 to 1 in 14 twist I believe I should be looking for? Any input much appreciated. :drinks: Oh, and to clarify, this is for .223

[Larry Gibson]

Any quality barrel for the AR is going to be "spendy" these days. However, if the goal is cast bullets then it's worth it in the long run. A couple years back I found a rebuilt M16 upper with a new milspec 12" twist barrel on it. I paid $250 for it. It not only is the most accurate AR/M16 I've ever used with M193 but it shoots my varmint loads into sub moa. What's even better is it really shines with cast and with .22LR using a M261 device.

Suggest you check with barrel makers instead of dealers. For strictly cast bullets I would look at a 20 - 24" 14" twist barrel. With several available cast bullets I would expect very useable accuracy in the 2400 - 2500+ fps range. With a .22LR device it would also be excellent and with 50-53 gr BTs at sevice velocity would do very well. With the 24" barrel you'd also pick up enough velocity so it should shoot M193 very well also.

Larry Gibson

[Houndog]

I agree with Larry on the 1/14 twist for a cast boolet only barrel, and if you want to shoot 55 GR FMJ BT bullets you will be fine. You can also get away with 60-63 FLAT BASE short ogive (6S or less) condom bullets, especially at higher altitudes, but If you live near sea level I'd use a 1/13 twist for a little more leway with the heavier condom bullets and you won't loose much over the 1/14 for cast.

If you are going to all the trouble to build a rifle DON"T SKIMP ON THE BARREL!!!!!! A first class barrel will cost you around $400! (Shielen select match, Douglas XX, Kreiger or a Bartlen in Stainless) An odd twist rate is no problem with any of the manufacturers listed. After you get a true match grade barrel, nothing else will satisfy you again. All those listed are straight, air guaged and hand lapped. Breakin is usually 10 or less rounds, and in fact I've won Benchrest matches with some of the ones listed with less than 50 rounds through the barrel.

My suggestion would be a 20-22" HBR stainless match barrel chambered with a Wilde (AR match) chamber in either 1/14 or 1/13 twist.

[Calamity Jake]

I've been thinking about building an AR, and i would like to cast for it if im gonna build it. From the research I've been able to do here, a slow twist barrel is a necessity, otherwise the barrel will lead horribly. The slowest twist I have been able to find on midwayusa is a 1 in 10" made by olympic, and it is quite spendy. Anyone know of a good source to pick up a barrel for an AR that is closer to the 1 in 12 to 1 in 14 twist I believe I should be looking for? Any input much appreciated. :drinks: Oh, and to clarify, this is for .223

My RRA heavy barrel 12 twist shoots like this a 100

http://photos.gunloads.com/images/CalamityJake/rralar15cast003.jpg

I think I can make it do better, just haven't had time to spermint with it.

[x101airborne]

So, just thinking (box) thinking,....... What if you got a varmint contour 223 rem barrel from a gunsmith as a take-off, would it be cost effective to have one turned, threaded, and chambered for use on the AR platform? I dont know anything about the diamaters required, but shortening a 24 inch factory barrel, threading for an adapter, and drilling a gas hole SHOULD in theory result in around a 21 inch 1-12 twist barrel. No one flame me if I am wrong, it is just an idea. I would even think that a "worn" barrel that has been cleaned would work well.

[nicholst55]

So, just thinking (box) thinking,....... What if you got a varmint contour 223 rem barrel from a gunsmith as a take-off, would it be cost effective to have one turned, threaded, and chambered for use on the AR platform? I dont know anything about the diamaters required, but shortening a 24 inch factory barrel, threading for an adapter, and drilling a gas hole SHOULD in theory result in around a 21 inch 1-12 twist barrel. No one flame me if I am wrong, it is just an idea. I would even think that a "worn" barrel that has been cleaned would work well.

If I was going to turn a barrel (blank) down, I'd start with a new, high-quality aftermarket blank. By the time you get it turned down, threaded for barrel extension, gas port drilled, chambered, you'd have so much in it that it would not be cost effective. Just MHO there.

An M16A1 'pencil' barrel might work, if you can find an affordable one. They're 1:12" twist, but they are currently in demand for building 'retro' rifles.

[Gswain]

Hmm, I have access to a very nice lathe, Wonder how much I could get a barrel blank for? I can justify spending 2-300 on a barrel, but 400-500 is just out of the question for me. I realize it would be fantastic for accuracy, but plain and simple, IM not that good yet lol.

[Idaho Sharpshooter]

I would just check with Aaron at Delta Company Arms. He is making barrels now I believe, and can just make what you want and fit it to your upper at his place. That is as close to a sub-moa guarantee as you can get in this lifetime. He also cuts the Wylde chamber as well as 223 commercial and 5.56x45mm milspec. In a couple weeks, I will be at his shop having him fit up a 300 BLK to my LAR pistol upper and a few other things for me.

Rich

[gee-gaw]

My friend Brad turns down used barrels and installs them on AR uppers.
pm me and I'll give you his info.
Wayne

[shotman]

dont drive the cast fast and they WILL work in a 1 in 9/10 stay at 1000fps or less . That is not 300 yd but at less than 100 works
dougless does the slow but as said pricey

[Moonie]

dont drive the cast fast and they WILL work in a 1 in 9/10 stay at 1000fps or less . That is not 300 yd but at less than 100 works
dougless does the slow but as said pricey

But they won't cycle an AR...

[madsenshooter]

I don't see anything wrong with using a used barrel. That's what I had Aaron do with a 6mm Obermeyer I got off gunbroker for $10. Bullet fit seems to be a higher priority than any perceived RPM threshold, but I'm not sure of my twist, haven't measured it, it just looks fast, and 5R to boot.

[Larry Gibson]

I don't see anything wrong with using a used barrel. That's what I had Aaron do with a 6mm Obermeyer I got off gunbroker for $10. Bullet fit seems to be a higher priority than any perceived RPM threshold, but I'm not sure of my twist, haven't measured it, it just looks fast, and 5R to boot.

So have you measured the twist? If it is an 8" twist instead of a 14" twist from a BR take off then that last group on the other thread is about 3". If it is an 8" twist then that is "useable" accuracy as I mentioned. If its a 14" twist then further load development should tighten the group. Not sure many would want to use a duplex load in and AR though, remains to be seen though.

Larry Gibson

[madsenshooter]

I'm going to do that when I have less other projects going on. Got to get one of my Krags ready for Camp Perry now. That last group was shot with about half the bullets having visible runout, easier to shoot them up than pull them down, more fun too. I shot a couple hundred of the duplex loads the other day, no problems. That was a benchrest match where you single load, and I had the gas system turned off, I've not tried it for function yet. Should anyone wish to try the duplex, please start with a smaller charge of booster and work up. Changes in the booster seem to effect velocity more than changes to the main charge. I decreased the main charge .8gr before the match, but velocity remained virtually the same, only 7fps difference on average.

[madsenshooter]

Got around to it, it is 1/8, someone else can calculate the RPM@2300fps. No leading with over 100rds, but I should note, I don't use WW, my boolits are cast of a BHN 23 lead based babbitt that I water drop. I should also note, with the 5R rifling in this caliber, each land is only .002 tall, and radiused, so there's no reason why someone with a 1/8 twist with a more conventional rifling can't get accuracy at a usable velocity. I believe it to be a matter of bullet fit, and the proper alloy for the pressures you generate. I don't doubt for a minute that it would be easier with a slower twist though. At 2500fps I was right back to spraying the bullets all over, but who knows what a couple thousandths more on the nose of my bullet will do. Saying there is an RPM threshold, as though it is a rule that cannot be broken, I just don't know. I agree it is a plausible theory. But like my theory of my bullet getting a wobble due to the 5 groove rifling, it's just a theory. If it's a rule, give me a number that I can't exceed. I know, that's not likely as bullet design would present a lot variables, as would alloy. This isn't an attack on the RPM threshold, nor is it a personal attack on you Larry, so don't take it as either. You've been quite helpful and I've learned a lot stuff from your posts, most especially those that have dealt with SEE and the pressure testing you've done. One always has to question things, and I am after all, just a beginner. My 1/8 @2300 was getting close to doing as well as a fellow with a benchrest rifle, probably a 1/14, who was shooting his linotype 6mm bullets at 2900fps. Light bullets and wind do not a good combination make for the benchrest game.

[Larry Gibson]

madsenshooter

"Saying there is an RPM threshold, as though it is a rule that cannot be broken, I just don't know. I agree it is a plausible theory."

I really thought we had gotten by that concept that the RPM threshold is a "limit" or "rule" that "can not be broken". I wonder how many times I have explained that it is not. The RPM threshold can be pushed higher. You do that with that alloy, the duplex load and shooting the AR with the gas shut off. The RPM threshold will vary from 120-140,000 RM using regular cast bullets and normal loads for them. The RPM threshold may be pushed upwards with specially designed bullets, different alloys (other than the regular WW - linotype alloys with BHNs of 12 -22) and slower burning powders just as you've done. Obviously the RPM threshold caught up with you at 2500 fps or so. `

I think the question here rides around the concept that the OP probably wants a cast bullet load for his AR that functions the action. Most AR owners aren't pleased with single shot ARs. So I would have to ask what loads do you have that function the action and show at least that amount of reasonable accuracy? Additionally, you know when the RPM threshold is catching up to your load when groups at 200 yards are non linear in expansion compared to 100 yard groups.

Larry Gibson

[9.3X62AL]

Calamity Jake......

That same RRA upper is the one I had in mind for my current EBR Rat Splasher. How does it do with the redcoated pretender bullets? Yours appears to have promise with the castings, for sure. Your group is about neck-and-neck with my bolter 223 and its 1-9" twist.

[madsenshooter]

I'm afraid I didn't read enough of of the RPM threads I came up with via searching, though I'd asked a couple times about RPM, twist rate, etc. No doubt RPM is a factor, just as Lee's ideas on pressure and alloys are a factor. It's a term you use and perhaps have done enough testing to see that RPM is what causes the inaccuracy for a given load. No doubt you took other factors into consideration before deciding to use the term. I have a rifle length gas tube and port, I'll check it out, but I feel the duplex will cycle my action. It will cycle a 6x45 with a carbine length gas system, I'm told. I know it takes about 24gr of powder more in line with what one would usually use in the 6x45 or 223 to cycle it with jacketed, and 4198 is too fast. I've no loads to help the original OP as I was initially stuck with using fast burners to get 1600fps. I am however of the opinion that slow burning ball powder like 2700 ought to work well with cast in either 223 or 6x45, wish I knew what mom did with that 8lbs that got lost in a move about 10yrs ago! 24gr of 4895 was the spraying 2500fps load, I imagine it would have cycled the action. Now that I know RPM threshold is just terminology, I'll worry about RPM when my boolits start blowing up. I'm outta this thread.

[Gswain]

In reguards to the RPM threshold, the simplest way to put that is the maximum amount of centrifugal force your boolit can endure before it spins itself apart. This is purely a function of
the alloy.

This is assuming you have proper boolit - bore fit:

This appears to be compounded by the "twist threshold", or the speed of twist in your barrel combined with the length of the barrel and how hot you are trying to load. As far as I can tell, the source of this threshold is the actual shear strength of the alloy you are shooting. Fast twist in a barrel means you impart a more aggressive spin to the boolit, meaning you have to either reduce your powder charge, or increase the shear strength of your alloy accordingly. A slower twist in a barrel would allow you to increase powder charge or decrease the shear strength of the alloy. The mechanism that is affecting all of this is the rifling grooves of the barrel attempting to rip the outer few thousandths off the boolit through rotational acceleration.

Sorry for the slight rambling with not much reason, just trying to summarize this for someone else that comes along and gets confused.

As for what I am looking for in a barrel, 1-10+ twist rate, and ideally firing loads that will cycle the action reliably. The primary reason I want the slow twist is so that I can get up to resembling regular jacketed performance, just shooting at 100 yards isn't quite as fun for me, and I'd like something that can reach out a bit more. Im hoping for the 60gr boolit at 2500 fps bracket, just for a rough idea.

I know the barrels made by shilen and such are fantastic, but Im a do it myself kinda guy, and if I was looking at spending that kind of money, I believe I would probably just try and find a blank barrel with the rifling I want, and turn it on my lathe, put a 223 wilde chamber in it. The closest I have seen was the RRA varmint barrel that calamity jake is using, I was hoping for something more like a 22" or 20" barrel though. I could possibly just cut that down.

Again, thanks for all the great input in here guys!

[Larry Gibson]

Yes this thread has gone a little off track. Too bad there is still a lot of misunderstanding about the RPM threshold. It's not "just terminology" and it's adverse affects occur a way before one needs to worry about "when boolits start blowing up".

Apparently Gswain has a good handle on it and that's why he's wanting a slower twist AR for shooting cast bullets. As far as standard AR twists the 12" will be the best if he can find one. But since he is building the rifle there are many 'smiths out there that can turn a standard 14" twist barrel to AR contours. The standard 20" barrel would do if Gswain wants a "handy rifle but if not a heavier 24" barrel would be better. The standard rifle length gas tube would work best for the milder pressured cast bullet loads. I also would get a military chamber for the M855 cartridge. This will allow the seating of 55 - 60 gr cast bullets so the GC stays in the case neck. One last thought; since this is a cast bullet AR consideration might be given to the .222. The .222 is easily formed from .223 cases, will feed from the magazine with no problems and the longer neck makes it more cast bullet friendly....just a thought.

The use of medium to slow burning powders from 4895 to AA4350 would betwhere I'd start with a 14" twist barrel. The best all around cast bullet for the .223 for me has been the 225462 Lovern style. The down side is the 2 front lube grooves are outside the case neck. Some years ago i found success with that bullet using H4895 in a 12" twist Colt AR at 22-2300 fps. That load has served me well in other 12" twist ARs and just so-so in 9" twist ARs over the years. I've 7, 9 and 12" ARs so I guess I'm going to have to revisit cast bullets in the .223 in the AR. I really don't expect the results to be much different that they are in my 9, 12 and 14" twist bolt guns though.

Larry Gibson

[Larry Gibson]

Gswain

While the centrifugal force of RPM can and does spin some bullets apart spinning bullets apart has nothing to do with the RPM threshold. The RPM threshold occurs at a point when;

the bullet is unbalanced or becomes unbalance due to obturation ing the bore during acceleration. The unbalanced bullet is forced to conform while in the barrel and it's center of mass is revolving around it's geometric center. When the bullet is free of the barrel's constraint, it will move in the direction that it's mass center had at the point of release. After exiting the muzzle, the geometric center will begin to revolve about the center of mass and it will depart at an angle to the bore (line of departure). At 54,000 RPM to 250,000 RPM, depending on velocity and twist, the centrifugal force can be tremendous. It will result in an outward or radial acceleration from the intended flight path (line of departure) and will try to get the bullet to rotate in a constantly growing helix.

"Try to" are the key words here as there are things we do (slower burning powders, harder alloys, better designed bullets, perfect fit, etc.) that we do to push the RPM threshold upward. Conversely, using a faster powder, softer alloy, no GC, etc) lowers the RPM threshold. When the bullet goes beyond the RPM threshold it does not lose "stability". The bullet still is flying point forword. It's flight path simply becomes a larger expanding helical one instead of a straight arch path that we expect. This is why when the RPM threshold is exceeded the groups expansion as range increases is non linear.

With cast bullets the RPM threshold will be exceeded long before the centrifugal force is enough to "spin the bullets apart". Exceeding the RPM threshold becomes apparent by the decrease in accuracy as velocity increases and the non linear expansion of group size as range increases.

Definatel do the 14" twist barrel if you are putting it on yourself. I think you'll be quite pleased. You should get into the 24-2500 fps range with pretty good accuracy (2 moa if the rifle and cast bullets are up to it) without too much problem. Above that it does get a little tricky and requires attention to detail a lot more.

Larry Gibson

[Gswain]

Thats some damn good tech there Larry, makes perfect sense too. I guess what I am trying to figure out is why a faster powder would have an effect on the center of mass vs. geometric center? I know in a perfect world, with our perfect boolit molds, everything should be perfectly balanced, so what is causing the difference between center of mass and geometric center? The reason I am stymied by this is I am thinking in comparison to jacketed bullets. It would seem from most load information that a jacketed bullet has a higher RPM threshold, at least based upon the speeds that they are loaded to. But if anything, a lead boolit should have a closer match between its geometric center and center of mass, due to being a tighter fit in the bore (if it is sized properly) than the jacketed round, which should generally be a couple thousandths smaller? Maybe im missing something here.

[madsenshooter]

Ah, "growing helix" now that's the term I was looking for when I said, in my country boy way, that five groove rifling produces a "wobble"! With luck, filling the space between the top of a land and the opposing groove with bullet nose will eliminate that. I'll see, ere long.

[Larry Gibson]

Gswain

The cast bullet is not contained by the jacket of a jacketed bullet or the PP of a patched bullet. Thus it is susceptable to all the stress during acceleration. These "stresses" cause obturation, sloughing and setback of the bullet. The nose of a bore rider can bend to one side if not fully suppoerted by the bore or even swage out into the grooves on one side. The bullet can collpase back onto itself into the lube grooves. The bullet can also be be swaged outward at the base before entering the throat and then has to be swaged back down again. The question is; do all or any of these happen evenly so the bullet stays balanced with the center of mass coinciding with the center of form? Probably not. For a given pressure with a specific bullet in a rifle the faster burning powder accelerates the bullet faster. Newton's Law that a body at rest tends to stay at rest is in affect here. Deformation can occur during accelleration as the bullet gets up to speed. A slower buring powder accellerates slower and would give less deformation of the bullet during accelleration.

The fast powder gives the bullet a "kick" and the slower burning powders give the bullet a "shove". That's a simplified analogy but you should get the idea.

The less the bullet is deformed or unbalance in the bore the less the centrifugal force will act on it and it can be driven to a higher velocity while maintaining reasonable accuracy.

Larry Gibson

[CWME]

Great posts Larry, thank you for sharing that. I have learned a lot from reading your posts.

[Calamity Jake]

Calamity Jake......

That same RRA upper is the one I had in mind for my current EBR Rat Splasher. How does it do with the redcoated pretender bullets? Yours appears to have promise with the castings, for sure. Your group is about neck-and-neck with my bolter 223 and its 1-9" twist.

RRA garontees 3/4 min @ 100 with them pretender bullets but I've never put one thru it, it's been cast from the get go.

[wiljen]

I don't get the faster powder reference either Larry. If F=MA and the bullet is of a set mass, assuming they both leave the barrel at the same velocity then the acceleration that occurs inside the barrel by definition is from 0 to that exit velocity. How does the force change if the mass doesn't and the acceleration is the same? Or is the thought that the counter productive forces on the bullet only occur within the first inch or so of travel ?

[felix]

Will, it's better to state that the AVERAGE acceleration would be the same at boolit exit if the velocity is the same. Accuracy is dependent on where the barrel is pointing at the moment of exit, which is a function of the EXACT acceleration at that point in the barrel. It gets squirrelly at that moment because of the definition of the boolit's exit point. We are talking microseconds? ... felix

[madsenshooter]

I think I get it. Faster powder=in general, more pressure, & more obturation. Though fast and slow powders could be loaded to produce the same peak pressure the more gradual build with a slow powder would produce less obturation and/or deformation. Simply put, Lee's theories on pressure, and his better results with slow vs fast powders.

[Larry Gibson]

I don't get the faster powder reference either Larry. If F=MA and the bullet is of a set mass, assuming they both leave the barrel at the same velocity then the acceleration that occurs inside the barrel by definition is from 0 to that exit velocity. How does the force change if the mass doesn't and the acceleration is the same? Or is the thought that the counter productive forces on the bullet only occur within the first inch or so of travel ?

My bad[smilie=b: The post should have said pressure instead of velocity. I have corrected the post.

The slower burning powders have a slower rise time to the pressure curve and a higher pressure to continue pushing on the bullets after peak pressure.

Load a .44 magnum with a 250 gr SWC with Bullseye to 35,000 psi and load the same bullet to the same 35,000 psi with 2400. Both have the same psi but which gives the higher velocity?

Load a .308W with a 168 MK with 2400 to 60,000 spi and load the same bullet to the same psi with 4895. Both have the same psi so which will give the higher velocity?

Obviously the slower burning powder in each case gives the higher velocity.

Hope that gives a bit better explanation other than my mistken use of the wrong word. Thanks for the "catch".

Larry Gibson

[wiljen]

Ok, that does make more sense now. I was trying to make it work with velocity and wasn't getting there.

[Larry Gibson]

If we think about it if the velocity were to be the same then the slower burning powder would have the lower pressure and gentler time/pressure curve.

Larry Gibson